ANALYSIS ON CRACKING MECHANISM OF FRACTURED ROCK MASSES UNDER UNIAXIAL COMPRESSION BASED ON THE TWO CRACKING CRITERIA

In this paper, with two cracking criteria, i.e., Rankine’s maximum tensile stress theory and Mohr-Coulomb shear failure theory for tensile and shear crack initiation respectively, the cracking mechanism of fractured rock masses under uniaxial compression was analyzed. Based on the analytical solution to stress field of single inclined elliptic fracture in an infinite rock mass under far-field load, the factors of the principal stresses concentration surrounding the fracture and the cracking functions, cracking positions and the crack initiation loads of the two cracking mechanism were calculated for different ratio of the smaller to the larger semi-axis k of the elliptical fracture and fracture dip angle α (the angle from the larger semi-axis of elliptic fracture to direction of the load) through a Matlab program. For rock masses containing multiple elliptic fractures, elastic stress fields were calculated by ABAQUS and cracking mechanisms were analyzed. It is concluded that: 1) With the increase of the number of fractures, the factors of the principal stresses concentration in the rock matrix increase slightly and the crack initiation loads decrease slightly, while the cracking positions are the same. 2) With the increase of fracture dip angle, the tensile stress concentration of the rock matrix changes from a small zone around fracture tip to a large zone surrounding the fracture, and vice versa for compressive stress, 3) There exists an critical fracture dip angle α0 which is around 45°; When 0<α≤α0, i.e., the fracture dip angle is small, the tensile stress and shear stress are both concentrated in the vicinity of fracture tip and the tensile crack initiation load are close to the shear crack initiation load with the increase of the ratio of the smaller to the larger semi-axis k, which means that tensile cracks and shear cracks will appear simultaneously; While for α0<α≤90°, i.e., the fracture dip angle is large, shear cracks or crushing zones will firstly initiate at the small region of the fracture tip due to high compressive stress concentration, and then large amount of tensile cracks will occur at the middle or tip of the pre-existing fractures due to high tensile stress concentration in this zone. The above analyses coincide with the experimental results.